The present invention relates to aqueous polymer dispersions comprising polyurethanes useful in coating compositions.
In producing water-based polyurethane resins for use in coating compositions, it is desirable to achieve a combination of properties including low temperature curing, gloss, flexibility, durability, abrasion resistance, and solvent resistance. A particular problem encountered with at least some water-based polyurethane coating compositions has been a susceptibility to loss of adhesion when the cured coating is exposed to humid conditions. The following prior art attempts at producing water-based polyurethanes do not address the humidity resistance problem.
U.S. Pat. No. 3,705,164 (Honig et al.) discloses a process for making stable aqueous polymer dispersions by subjecting vinyl monomers (e.g., acrylics) to emulsion polymerization in the presence of a stable aqueous dispersion of a polyurethane containing anionic groups. Coatings are disclosed as a use of the resulting polymer dispersions.
U.S. Pat. No. 4,198,330 (Kaizerman et al.) discloses modifying an aqueous polyurethane by means of vinyl monomers by a process similar to that of the U.S. Pat. No. 3,705,164 patent.
U.S. Pat. No. 4,644,030 (Loewrigkeit et al.) discloses a method for making an aqueous dispersion of polyurethane by first producing an isocyanate-terminated polyurethane prepolymer in ethylenically unsaturated monomers such as acrylates. The polyurethane prepolymer is then dispersed in water and is chain-extended. Thereafter the unsaturated monomer material is polymerized in situ in the aqueous dispersion.
EP-A-0 308 115 discloses an aqueous polymer dispersion containing an anionic water-dispersible polyurethane and a vinyl polymer obtained by a process similar to that of the U.S. Pat. No. 4,644,030 patent.
U.S. Pat. No. 4,318,833 (Guagliardo) discloses a waterborne polyurethane/acrylate in which the acrylate portion is dominant.
U.S. Pat. No. 4,791,168 (Salatin et al.) relates to incorporating polyester polyols made from long-chain carboxylic acids into waterborne polyurethane resins. The polyesters may be made by reacting alcohols with C.sub.36 dimer acid sold under the name EMPOL 1010. The patent discloses that the long chain acids add hydrophobicity to the polyester to achieve the objective of increasing the speed with which waterborne polyurethane coating compositions can be dried.
U.S. Pat. No. 5,118,752 (Chang et al.) discloses aqueous polymerization of vinyl monomers, including acrylates, in the presence of a polymeric surfactant containing urethane groups and silane groups. Among the isocyanates used to make the urethane portion of the surfactant is included a hydrophobic isocyanate ("DDI" diisocyanate). A comparative example without silane groups is also disclosed, but is reported in the patent to perform poorly. All of the examples disclosed in the patent employ a substantially larger amount of vinyl polymer relative to the polyurethane polymer, and the vinyl portion is predominately styrene.
U.S. Pat. No. 5,173,526 (Vijayendran et al.) involves a method for making aqueous polyurethane/acrylates similar to Chang et al., except that no silane groups are included, no hydrophobic isocyanates are used, and the use of an oil-soluble initiator is required. The resulting aqueous polymer dispersion is intended for use in paper coatings.
Commonly owned, copending U.S. patent application Ser. Nos. 08/668,069 and 08/668,901 relate to polyurethane and polyurethane/acrylic polymers having improved humidity resistance provided by the inclusion of a polyurethane synthesized from a polyisocyanate having at least one open carbon chain greater than six carbon atoms in length between two isocyanate groups, preferably at least eight carbon atoms in length. Although this polyisocyanate preferably constitutes a minor portion of the polyisocyanate mixture used to make the polyurethane, commercially available polyisocyanates of this type are relatively costly. It would be desirable to improve humidity resistance properties without incurring the high cost of the prior applications.